Numerous techniques have been proposed for manufacture of microspheres of plasticizable material such as glass and polymers. The disclosures of the U.S. Pat. Nos. to Budrick et al 4,017,290 and 4,021,253, and to Hendricks 4,133,854 are exemplary. For glass microspheres, a dried gel powder or frit with an occluded blowing agent may be dropped through a tower furnace to form a microsphere having internally trapped residual gases or liquid. Similarly, it has been proposed as in Veatch et al U.S. Pat. No. 2,797,201 that polymeric shells may be formed by dissolving a film forming polymer in a suitable volatile solvent and then spraying the solution into the upper portion of a heated chamber.
In order to achieve greater shell size during the blowing or forming operation, it has been proposed as in the above-noted Hendricks patent to "pull a vacuum" or substantially reduce the ambient pressure in the heated volume of the chamber or furnace. However, furnace size, sometimes on the order of four meters or more, renders such an operation difficult to accomplish and control in practice, and makes unreliable efforts to control the dimensional parameters of the ultimate shells. Similar problems inhere to a greater or lesser extent in other types of operations for blowing glass or polymeric shells at elevated temperature in appreciable quantities.